Devices with flame retardant ultraviolet cured multi-layered film

ABSTRACT

A flame resistant composite film includes a first layer and a second layer that are joined together. The first layer has a light transmission of at least 93 percent and is substantially constructed from a radiation-cured polymer and a flame retardant material. The second layer is constructed primarily from thermoplastic polymer resin. The first and second layer when taken as a whole have a flame retardancy sufficient to pass VTM-2 standard of Oct. 27, 1988, of Underwriters&#39; Laboratories, Inc. Also, microstructured films comprising such flame retardant films, backlit displays comprising such microstructured films, and computers comprising such displays.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 08,383,064,filed Feb. 3, 1995, issued as U.S. Pat. No. 5,855,983.

BACKGROUND OF THE INVENTION

The present invention relates generally to a flame retardantmulti-layered structure. More particularly, the present inventionrelates to a flame retardant microstructure bearing multi-layeredstructure.

The use of a microstructure bearing article as a brightness enhancementfilm is known in the art. Illustrative brightness enhancement films aredescribed in Whitehead, U.S. Pat. No. 4,542,449; Lu et al., U.S. Pat.No. 5,175,030; and Lu, U.S. Pat. No. 5,183,597. The Lu et al. and Lupatents disclose microstructure bearing composite polymeric articles anda method of forming microstructure bearing composite polymeric articles.The Lu et al. and Lu patents disclose forming the microstructure withdesired optical properties, such as total internal reflection.

Microstructure bearing articles are made in a variety of forms. One suchform includes a series of alternating tips and grooves. One example ofsuch a form is brightness enhancement film, which has a regularrepeating pattern of symmetrical tips and grooves. Other examplesinclude patterns in which the tips and grooves are not symmetrical andin which the size, orientation, or distance between the tips and groovesis not uniform. When the included angle of the grooves is in the rangeof about 70° to about 120° and especially within 80° to about 100°, thearticle with alternating tips and grooves is useful as a brightnessenhancement film.

Brightness enhancement film is typically incorporated into a liquidcrystal display, which is then incorporated into an electronic devicesuch as a lap-top computer. Devices such as lap-top computers arerequired to have a flammability resistance rating. For making lap-topcomputers, manufacturers prefer to use components that have anestablished flammability resistance rating. If all the components in alap-top computer have a satisfactory flammability resistance rating, themanufacturer of the lap-top computer doe snot have to submit theassembled lap-top computer for a flammability resistance rating.

Prior art brightness enhancement films include "Brightness EnhancementFilm", a version made from polycarbonate thermoplastic, sold byMinnesota Mining and Manufacturing Company, St. Paul, Minn. and "DIAART"a version made of a radiation cured microstructured layer on a polyesterbase, sold by Mitsubishi Rayon, Tokyo, Japan.

Conventional prior art brightness enhancement films manufactured frompolycarbonate and radiation-cured composites, however, do not exhibitsufficient flammability resistance characteristics to qualify for thelowest flammability rating from a recognized rating organization, suchas Underwriters' Laboratories, Inc. There are several difficulties thatmust be overcome when constructing a brightness enhancement film withflammability resistance characteristics.

Prior art flame retardant chemicals that are used to reduce theflammability of polymers are typically insoluble salts, such as antimonyoxide, or highly crystalline compounds, such as decabromobiphenyl. Thesecompounds are not suitable for use with radiation curable resins, whichare preferred for marking brightness enhancement film. Such compoundsinhibit radiation curing of the resin or reduce the desired opticalqualities of the film.

SUMMARY OF THE INVENTION

The present invention provides a flame resistant multi-layered filmcomprising a first layer and a second layer that are joined together.The first layer has a light transmission of at least 93 percent and isconstructed substantially from a radiation-cured polymer and a flameretardant material. The second layer is constructed primarily from athermoplastic polymer resins. The first and second layers when taken asa whole have a flame retardancy sufficient to pass the VTM-2 standard ofOct. 27, 1988, of Underwriters' Laboratories, Inc. The present inventionalso provides backlit displays comprising such articles, and computerscomprising such displays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an illustrative film of the presentinvention in a backlit liquid crystal display.

FIG. 2 is a perspective view of an illustrative microstructure bearingpolymeric article of the present invention.

These figures, which are not to scale, are intended to be merelyillustrative and non-limiting.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention provides a flame retardant, multi-layered filmthat is suitable for use as a brightness enhancement film. Themulti-layered film preferably contains a relatively thick backing layer,sometimes referred to herein as a "second layer", and a relatively thinradiation-cured layer, sometimes referred to herein as a "first layer",that is formed into a microstructure suitable for a desired application.

An illustrative backlit liquid crystal display generally indicated at 10in FIG. 1 includes a brightness enhancement film 11 of the presentinvention which is typically positioned between a diffuser 12 and aliquid crystal display panel 14. The backlit liquid crystal display alsoincludes a light source 16 such as a fluorescent lamp, a light guide 18for transporting light for reflection toward the liquid crystal displaypanel 14, and a white reflector 20 for reflecting light also toward theliquid crystal display panel. The brightness enhancement film 11collimates light emitted from the light guide 18 thereby increasing thebrightness of the liquid crystal display panel 14. The increasedbrightness enables a sharper image to be produced by the liquid crystaldisplay panel and allows the power of the light source 16 to be reducedto produce a selected brightness. The brightness enhancement film 11 inthe backlit liquid crystal display is useful in equipment such ascomputers, personal televisions, video recorders, mobile communicationdevices, and automobile and avionic instrument displays, represented byreference character 21. The brightness enhancement film 11 includes anarray of prisms typified by prisms 22, 24, 26, and 28, as illustrated inFIG. 2. Each prism, for example, such as prism 22, has a first facet 30and a second facet 32. The prisms 22, 24, 26, and 28 are formed on abody portion 34 that has a first surface 36 on which the prisms areformed and a second surface 38 that is substantially flat or planar andopposite the first surface.

A linear array of regular right prisms is preferred for both opticalperformance and ease of manufacture. By right prisms, it is meant thatthe apex angle Θ is typically 90°, but can also range from 70° to 120°and most preferably from 80° to 100°. The prism facets need not beidentical, and the prisms may be tilted with respect to each other.Furthermore, the relationship between the thickness 40 of the film andthe height 42 of the prisms is not critical, but it is desirable to usethinner films with well defined prism facets. A typical ratio of prismheight 42 to total thickness 40 is generally between 25/125 and 2/125.

The angle that the facets would form with the surface 38 if the facetswere to be projected would typically be 45°. However, this angle wouldvary depending on the pitch of the facet or the angle Θ of the apex.

The multi-layered structure according to the present invention has anadditional benefit of exhibiting sufficient flame retardancecharacteristics to qualify for Underwriters' Laboratories, Inc.'s lowestflammability rating of VTM-2 dated Oct. 27, 1988. As a result of theVTM-2 rating, a manufacturer who incorporates the present brightnessenhancement film into a device with other components, which all have theVTM-2 rating, can avoid the time and expense of certifying that theentire device has a VTM-2 rating.

Neither the microstructured layer nor most of the backing materials usedin prior art brightness enhancement films consistently meet the VTM-2standard.

The microstructure bearing film is preferably formed according to aprocess similar to the processes disclosed in Lu et al., U.S. Pat. No.5,175,030, and Lu, U.S. Pat. No. 5,183,597. The formation processpreferably includes the following steps:

(a) preparing an oligomeric resin composition;

(b) depositing the oligomeric resin composition onto a master negativemicrostructured molding surface in an amount barely sufficient to fillthe cavities of the master; and

(c) filling the cavities by moving a bead of the composition between apreformed backing and the master, at least one of which is flexible.

(d) curing the oligomeric composition and removing the microstructurebearing article to form the master.

The oligomeric resin composition of step (a) is a one-part,solvent-free, radiation-polymerizable, cross-linkable, organicoligomeric composition. In one embodiment, the oligomeric composition isa blend of a radiation curable component and at least one non-acrylateflame retardant chemical. A photoinitiator may also be used, typicallyin a concentration of about approximately 0.1 to 1.0 percent by weightof the oligomeric composition. Higher concentrations of thephotoinitiator can be used but are not normally needed to obtain thedesired properties.

The radiation curable portion of the oligomeric composition comprisesbetween about 70 and 100 percent by weight of the oligomeric compositionand may contain more than one component. At least one of the componentsis preferably brominated. In some embodiments the polymerized oligomericcomposition contains at least approximately 10 percent of a brominesubstituted component.

Components that provide a high refractive index material are especiallysuitable for use in the radiation-cured polymers. Examples of suchcomponents along with their preferred trade name and manufacturerinclude: bisphenol-A epoxy diacrylate (PHOTOMER Brand 3016, HenkelCorporation of Ambler, Pa.), phenoxyethyl acrylate (PHOTOMER Brand 4035,Henkel Corporation of Ambler, Pa.), tribromophenoxyethyl acrylate(BR-31, Dai-Ichi Chemical of Elk Grove Village, Ill.),tetrabromobisphenol-A diacrylate (SR-640, Sartomer Company of Exton,Pa.), benzyl acrylate (SR-432, Sartomer Company of Exton, Pa.), andtetrabromobisphenol-A ethoxylated diacrylate (RDX 51027, UCB-Radcure ofSmyrna, Ga.). Examples of other components which can be used but whichgive a lower refractive index include: neopentylglycol propoxylatediacrylate (PHOTOMER Brand 4127, Henkel Corporation of Ambler, Pa.),polyether-based urethane acrylate oligomer (EBECRYL Brand 4826,UCB-Radcure of Smyrna, Ga.), polyester-based urethane acrylate oligomer(PHOTOMER Brand 6010, Henkel Corporation of Ambler, Pa.).

It is known in the art that combinations of bromine-containing compoundsand phosphorous-containing compounds give useful flame retardantproperties. Such combinations are typically used to renderthermoplastics, such as polycarbonate or polystyrene, flame retardant.However, when such compounds are mixed with (meth)acrylic-functionalcompounds useful in radiation curable compositions for optical devices,the resulting mixtures are not clear solutions. Even when the uncuredmixtures are clear solutions, the cured articles are usually notoptically clear. This result is common when radiation curable andnon-radiation curable components are mixed and cured with radiation suchas ultraviolet or visible light. For the manufacture of optically cleararticles by radiation curing, it would be advantageous to employ onlycomponents which have (meth)acrylate functionality, such that thecomponents would react with each other and give a clear, colorlesspolymer. Preferably, the clear colorless polymer has a sufficiently lowabsorption coefficient that no more than 7 percent of the light incidenton the first layer is lost by absorption. However, a significant portionof the transmitted light may be internally reflected by the external(particularly when faceted) surface of the first layer or reflected byother surfaces and interfaces of the film, resulting in net lighttransmission lower than 93 percent. The term "light transmission rate ofat least 93 percent" is defined to mean that the percentage of lightwhich is not absorbed as the light is transmitted through the maximumthickness of the first layer is at least 93 percent of the lightincident on the first layer.

This invention embodies the discovery that some non-acrylate,phosphorus-containing flame retardant chemicals have both enoughsolubility and enough flame retardant properties to be useful infabricating flame retardant microstructured optical films. By"non-acrylate" it is meant that the chemicals do not contain anysignificant amount of (meth)acrylate groups and are not consideredradiation-polymerizable by those skilled in the art. Suchphosphorus-containing flame retardant chemicals have a low tendency tobleed or be exuded from the cured resin and are preferred. Illustrativeexamples of such chemicals along with their preferred trade name andmanufacturer include: cyclic phosphonate ester (AB-19, Albright andWilson of Richmond, Va.), halogenated phosphate ester (HP-36, GreatLakes Chemical Corporation of West Lafayette, Ind. and PB 460, FMCCorporation of Princeton, N.J.). Typically the flame retardant chemicalconstitutes less than 30 weight percent, preferably less than 10 weightpercent of the oligomeric composition.

The viscosity of the oligomeric composition deposited in step (b) ispreferably between 1,000 centipoise (1.0 pascal-seconds) and 5,000centipoise (5.0 pascal-seconds). If the oligomeric composition has aviscosity above this range, air bubbles might become entrapped in thecomposition. Additionally, the composition might not completely fill thecavities in the master. When an oligomeric composition with a viscositybelow that range is used, the oligomeric composition usually tends toexperience shrinkage upon curing that prevents the oligomericcomposition from accurately replicating the master.

Polymerization can be accomplished by usual means, such as heating inthe presence of free radical initiators, irradiation with ultraviolet orvisible light in the presence of suitable photoinitiators, and byirradiation with electron beams. For reasons of convenience, low capitalinvestment, and production speed, the preferred method of polymerizationis by irradiation with ultraviolet or visible light in the presence of aphotoinitiator at a concentration of about 0.1 percent to about 1.0percent by weight of the oligomeric composition. Higher concentrationscan be used but are not normally needed to obtain the desired curedresin properties. The curing conditions and the processes disclosed inU.S. Pat. Nos. 5,175,030 and 5,183,597 are considered satisfactory toprepare the microstructure bearing article of this invention.

The ideal backing of the flame retardant microstructure bearing articlewould be a backing which was by itself flame retardant. However, knownpolymeric films which are sold as "flame retardant" are not opticallyclear enough to be useful in optically functional microstructure bearingarticles, such as a brightness enhancement film. Polyvinylchloride film,for example, is inherently flame retardant, but it is not available inthe optical quality needed for such a film to be useful in a brightnessenhancement film. Therefore, commonly available, clear polymeric filmsare preferred.

Polyethylene terephthalate or polycarbonate film are preferable for useas a backing in step (c) because the materials are economical, opticallyclear, and have good tensile strength. Thickness of 0.025 millimeter to0.5 millimeter are preferred, and thickness of 0.075 millimeter to 0.175millimeter are especially preferred.

Other useful backings for the microstructure bearing articles includecellulose acetate butyrate, cellulose acetate propionate, polyethersulfone, polymethyl methacrylate, and polyurethane. Glass is also usefulin certain articles, but does not have the flexibility and toughnessavailable that the polymeric films mentioned above have. The surface ofthe backing may also be treated to promote adhesion to the oligomericcomposition.

Polyethylene terephthalate based materials having good optical qualitiesand acceptable adhesion are preferred. Examples of such polyethyleneterephthalate based materials include: a photograde polyethyleneterephthalate; a polyethylene terephthalate (PET) having a surface thatis formed according to the method described in U.S. Pat. No. 4,340,276(Example 5), referred to herein as "SEP-PET"; and MELINEX PETmanufactured by ICI Films of Wilmington, Del. The SEP-PET has a materialsuch as chromium applied in an amount which results in a compositesurface on which portions of the underlying PET are exposed betweendiscontinuous microislands of the material. The composite surface issputter etched with an oxygen plasma to preferentially etch the exposedportions of the higher sputtering rate PET, while the discontinuousmicroislands are etched at a lower rate, resulting in a topography ofmicropedestals which vary in height within a range of approximately 0.01and 0.2 μm and which are separated from adjacent micropedestals adistance within a range of approximately 0.05 to 0.5 μm.

Preferred polycarbonate based materials and manufacturers include: PCEE600, a specially-formulated polycarbonate film, that was formerlyobtainable from Miles Inc. of Pittsburgh, Pa. but no longer available;MILES Brand PCEE manufactured by Miles Inc. of Pittsburgh, Pa.; LEXANBrand 8050 manufactured by GE Plastics Company of Pittsfield, Mass.Polycarbonate film that is modified to be flame retardant is availablefrom GE Plastics Company. However, it is more expensive than unmodifiedpolycarbonate film, tends to have higher optical absorption, and is notavailable in thicknesses below 0.25 millimeter.

A cross-linking agent may also be added to the oligomeric resincomposition to further aid in the cross-linking of the polymer.Illustrative examples of suitable cross-linking agents include:1,4-butylene di-methacrylate or -acrylate; ethylene di-methacrylate or-acrylate; trimethylolpropane di- or tri-acrylate; glyceryl di-acrylateor -methacrylate; glyceryl tri-acrylate or -methacrylate; glycidylacrylate or methacrylate; pentaerythritol triacrylate ortrimethacrylate; diallyl phthalate;2,2-bis(4-methacryloxyphenyl)-propane; diallyl adipate;di(2-acryloxyethyl) ether; dipentaerythritol pentacrylate; neopentylglycol diacrylate or dimethacrylate; polypropylene glycol diacrylate ordimethacrylate; 1,3,5-tri-(2-methacryloxyethyl)-s-triazine;hexamethylene diacrylate or dimethacrylate; poly(ethylenicallyunsatureated alkoxy)heterocycles, as taught in U.S. Pat. No. 4,262,072;and adducts of poly(isocyantes) with hydroxy- and amino- functionalacrylates, such as EB220 sold by UCB-Radcure, Smyrna, Ga. Cross-linkingagents used to make polymers with a refractive index of greater than1.56, such as EB220, are especially useful.

A preferred master for use with the above described method is a metallicmaster, such as nickel, nickel-plated copper or brass. If thetemperature of the curing and optional simultaneous heat treating stepis not too great, the master can also be constructed from athermoplastic material, such as a laminate of polyethylene andpolypropylene.

After the oligomeric resin fills the cavities between the backing andthe master, the oligomeric resin is cured and heat treated, if desired.The cured multi-layered structure is then removed from the master.

Samples of the multi-layered structure are then examined for flameretardance characteristics. Flame retardance is measured according toVTM-2 procedure dated Oct. 27, 1988 set forth by Underwriters'Laboratories.

The results of the test performed on examples designed to illustrate theinvention are reported in Table 1. Table 1 contains examples of coatedand uncoated structures. Composition amounts are expressed in percent byweight.

                                      TABLE 1                                     __________________________________________________________________________              Backing                        Coating                              Spl.      Thickness                                                                          Photomer                                                                            RDX                 Thickness                            No.                                                                              Backing                                                                              (10.sup.-3 mm)                                                                     3016                                                                             4035                                                                             51027                                                                             BR-31                                                                             AB-19                                                                             AB-100                                                                            HP-36                                                                             (10.sup.-3 mm)                                                                      VTM-2                          __________________________________________________________________________     1 PCEE 600                                                                             250                            none  pass                            2 PCEE 600                                                                             250  60%                                                                              40%                    88-125                                                                              fail                            3 LEXAN 8050                                                                           250                            none  fail                            4 MILINEX PET                                                                          125                            none  fail                            5 MILINEX PET                                                                          125        33% 67%             25-37 fail                            6 MILINEX PET                                                                          125     40%                                                                              60%                 50    fail                            7 MILINEX PET                                                                          125     40%                                                                              60%                 63    fail                            8 MILINEX PET                                                                          125     40%                                                                              60%                 63-88 BEF                                                                           fail                            9 MILINEX PET                                                                          125     36%                                                                              54%     10%         25    pass                           10 MILINEX PET                                                                          125     36%                                                                              54%     10%         63    pass                           11 MILINEX PET                                                                          125     36%                                                                              54%     10%         88-100                                                                              pass                           12 MILINEX PET                                                                          125     36%                                                                              54%     10%         50-63 BEF                                                                           pass                           13 MILINEX PET                                                                          125     36%                                                                              54%         10%     25-37 pass                           14 MILINEX PET                                                                          125     36%                                                                              54%         10%     50    pass                           15 MILINEX PET                                                                          125     36%                                                                              54%         10%     63-75 pass                           16 MILINEX PET                                                                          125     36%                                                                              54%             10% 25-37 fail                           17 MILINEX PET                                                                          125     36%                                                                              54%             10% 50    pass                           18 MILINEX PET                                                                          125     36%                                                                              54%             10% 100-125                                                                             pass                           19 MILINEX PET                                                                          125     36%                                                                              54%             10% 37-50 BEF                                                                           pass                           20 MILES PCEE                                                                           125                            none  ps/fl                          21 MILES PCEE                                                                           125  60%                                                                              40%                    63 BEF                                                                              fail                           22 MILES PCEE                                                                           125  60%                                                                              40%                    25 BEF                                                                              fail                           23 MILES PCEE                                                                           125     40%                                                                              60%                 75 BEF                                                                              fail                           24 MILES PCEE                                                                           125     40%                                                                              60%                 50-63 pass                           25 MILES PCEE                                                                           125     40%                                                                              60%                 88-125                                                                              pass                           26 MILES PCEE                                                                           125     40%                                                                              60%                 113-125                                                                             pass                           27 MILES PCEE                                                                           125     36%                                                                              54%     10%         37-50 fail                           28 MILES PCEE                                                                           125     36%                                                                              54%     10%         63-75 pass                           29 MILES PCEE                                                                           125     36%                                                                              54%     10%         88    pass                           30 MILES PCEE                                                                           125     36%                                                                              54%         10%     63-88 ?                              31 MILES PCEE                                                                           125     36%                                                                              54%         10%     88-100                                                                              ?                              32 MILES PCEE                                                                           125     36%                                                                              54%         10%     113-138                                                                             fail                           33 MILES PCEE                                                                           125     36%                                                                              54%             10% 50    fail                           34 MILES PCEE                                                                           125     36%                                                                              54%             10% 63-75 ?                              35 MILES PCEE                                                                           125     36%                                                                              54%             10% 75-100                                                                              fail                           36 MILES PCEE                                                                           125     45%    45% 10%         50-63 ?                              37 MILES PCEE                                                                           125     50%    50%             50-125                                                                              ?                              38 MILES PCEE                                                                           125        33% 67%             37    pass                           39 MILES PCEE                                                                           125  14%                                                                              29%    57%             25-37 pass                           40 MILES PCEE                                                                           125  50%.sup.1 50%             50-88 fail                           41 MILES PCEE                                                                           125  80%.sup.2     20%         88-100                                                                              fail                           42 MILES PCEE                                                                           125  80%.sup.3     20%         75    fail                           43 MILES PCEE                                                                           125     16%.sup.4                                                                        37%     47%.sup.3   37-63 BEF                                                                           fail                           44 3M PET  88  48%                                                                              32%                20% 88    fail                           45 3M PET 100     40%                                                                              60%                 50    fail                           46 3M PET 100     36%                                                                              54%     10%         12-25 fail                           47 3M PET 100     36%                                                                              54%         10%     25    fail                           48 3M PET 100     36%                                                                              54%             10% 25-37 fail                           49 3M PET 100     32%                                                                              48%     20%         75-88 pass                           50 3M PET 100     32%                                                                              48%         20%     75-88 pass                           51 3M PET 100     32%                                                                              48%             20% 88    pass                           52 SEP-PET                                                                              113                            none  fail                           53 SEP-PET                                                                              113        33% 67%             50-100 BEF                                                                          pass                           __________________________________________________________________________     .sup.1 PHOTOMER Brand 4127 resin                                              .sup.2 EBECRYL Brand 4826 resin                                               .sup.3 PHOTOMER Brand 6010 resin                                              .sup.4 SR640 resin                                                            .sup.5 SR432 resin                                                       

Entries in the Coating Thickness column with a "BEF" indicate that thecomposite structure was a brightness enhancement film, i.e., had amicrostructured surface as opposed to just being a two layer laminate.Entries in the VTM-2 column indicate the results obtained fromperforming that test procedure. For each sample, a battery of fivespecimens was tested. In accordance with the test procedure, if a givenspecimen burns for more than 30 seconds and/or burns for more than 5inches (12.5 cm) from the bottom, the specimen was deemed a failure. Insome instances, certain samples were tested again later, e.g., to verifystandards. The results are tabulated as follows: "pass" indicates thatwhen a battery of five specimens were tested all five passed, or one ofthe first battery of five specimens failed and then all five in a secondbattery of five passed; "?" indicates that at least one specimen failedin a battery of five specimens and no other specimens were tested;"ps/fl" indicates that all five specimens in the first battery of fivepassed but that one specimen of a second battery of five failed;"failed" indicates that at least one specimen failed in a first batteryof five, and at least one specimen failed in a second battery of five.

As illustrated by Samples 9-19, 27-35, and 46-51, the multi-layeredstructures that contain a relatively high percentage of brominatedcomponent(s) along with a phosphorous-based flame retardant provide thebest flame retardant characteristics. However, when examining Samples 9,10, 27, 28, 46, and 49, it can be seen flame retardant concentration andmicrostructure resin thickness needed to provide satisfactoryflammability resistance is dependent upon the backing material used inthe multi-layered structure. With the MILES PCEE backing used in Samples24-26, a sufficiently thick layer of halogenated resin containingmicrostructure without a flame retardant can result in a multi-layeredstructure that meets the VTM-2 standard.

However, a thick microstructured layer is not desirable because themicrostructured layer should not inhibit the flexibility of thecomposite structure. For these resins, which are preferable forbrightness enhancement film, a microstructured layer with thicknessgreater than about 20 percent of the backing thickness typically tendsto cause curling of the composite structure after curing. In addition,if the microstructure layer gets hard and thick, it tends to flake orchip off the backing when the backing is bent. Thus, it is desirable forboth the backing and the coating to be as flame retardant as possible,so that the multi-layered structure will pass the VTM-2 standard witheven a thin coating.

If the coating contains a sufficient concentration of brominated resin,as in Samples 5, 38 and 53, the composite can pass the VTM-2 standard ifthe coating is thick enough and the right backing is used.Unfortunately, this particular microstructured material doe snot stickto any backing except specially primed SEP-PET. As illustrated inSamples 41 and 42, even a relatively high amount of flame retardant anda thick coating cannot overcome the flammability of non-brominatedresins.

The multi-layered structure illustrated in Samples 12, 19, and 53 areexamples of flame retardant brightness enhancement film. Such abrightness enhancement film, when incorporated into a portableelectronic device with other VTM-2 rated materials, would enable themanufacturer of the portable electronic device to avoid the time andexpense of undergoing flammability resistance testing of the entiredevice.

Based on the results in Table 1, it can be seen that the propercombination of acrylate-functional resin components, non-acrylatefunctional flame retardant, and backing provide a brightness enhancementfilm that meets the VTM-2 requirements. Table 1 also illustrates howsubtle changes in the microstructured layer composition can make adifference between passing and failing the VTM-2 test. It can also beseen that if a ultraviolet radiation-cured resin microstructured layeris sufficiently thick and flame retardant, it can cause the entirecomposite structure to pass the VTM-2 test.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A backlit display comprising a light source;adisplay panel; and a multilayered article disposed between the lightsource and the display panel, the article comprising:a first and asecond layer joined together, the first layer being substantiallycomposed of a polymerized (methacrylic-functional oligomeric compositionand a flame retardant chemical and having a light transmission rate ofat least 93 percent, and the second layer substantially composed ofthermoplastic polymer, wherein the first and the second layers have acombined flame retardancy sufficient to pass VTM-2 standard of Oct. 27,1988, of Underwriters' Laboratories, Inc.
 2. The backlit display ofclaim 1 wherein the display panel is a liquid crystal display panel. 3.The backlit display of claim 1 wherein the first layer is thinner thanthe second layer.
 4. The backlit display of claim 3 wherein said secondlayer is between 0.025 and 0.5 millimeter thick.
 5. The backlit displayof claim 3 wherein the thickness of said first layer is not greater than20 percent of the thickness of said second layer.
 6. The backlit displayof claim 1 wherein the flame retardant chemical is a non-acrylate. 7.The backlit display of claim 6 wherein the non-acrylate flame retardantchemical used in the first layer is cyclic phosphonate ester,halogenated phosphate ester, or combinations thereof.
 8. The backlitdisplay of claim 7 wherein said first layer is the polymerized productof an oligomeric composition comprising less than 30 weight percent ofsaid flame retardant chemical.
 9. The backlit of claim 7 wherein saidfirst layer is the polymerized product of an oligomeric compositioncomprising less than 10 weight percent of said flame retardant chemical.10. The backlit display of claim 1 wherein the polymerized oligomericcomposition contains at least approximately 10 percent of a brominesubstituted component.
 11. The backlit display of claim 1 wherein thepolymerized oligomeric composition used in the first layer comprises atleast one of the following: bisphenol-A epoxy diacrylate, brominatedbisphenol-A epoxy diacrylate, phenoxyethyl acrylate,tribromophenoxyethyl acrylate, tetrabromobisphenol-A diacrylate, benzylacrylate, tetrabromobisphenol-A ethoxylated diacrylate, or combinationsthereof.
 12. The backlit display of claim 1 wherein the second layer isformed from polyethylene terephthalate or polycarbonate.
 13. The backlitdisplay of claim 1 wherein said first layer further comprises across-linking agent.
 14. The backlit display of claim 13 wherein thecross-linking agent comprises at least one of the following:1,4-butylene di-methacrylate or -acrylate; ethylene di-methacrylate or-acrylate; trimethylolpropane di- or tri-acrylate; glyceryl di-acrylateor -methacrylate; glyceryl tri-acrylate or -methacrylate; glycidylacrylate or methacrylate; pentaerythritol triacrylate ortrimethacrylate; diallyl phthalate;2,2-bis(4-methacryloxyphenyl)-propane; diallyl adipate;di(2-acryloxyethyl) ether; dipentaerythritol pentacrylate; neopentylglycol diacrylate or dimethacrylate; polypropylene glycol diacrylate ordimethacrylate; 1,3,5-tri-(2-(meth)acryloxyethyl)-s-triazine;hexamethylene diacrylate or dimethacrylate; poly(ethylenicallyunsaturated alkoxy) heterocycles; adducts of poly(isocyanates) withhydroxy- and amino- functional acrylates, or combinations thereof. 15.The backlit display of claim 1 wherein the second layer has a sputteretched surface adjacent to the first layer.
 16. The backlit display ofclaim 1 wherein a surface of the first layer has microstructures formedtherein.
 17. The backlit display of claim 1 further comprising aphotoinitiator.
 18. The backlit display of claim 1 wherein said flameretardant material is not chemically bound to said polymerizedoligimeric composition.
 19. A computer comprising a backlit display ofclaim 1.